The present invention relates to an expressing filter plate for filter presses of the expression type.
Various types of expression filter presses are known which include one type comprising base plates and diaphragm sheets each covering the base plate and having a filter surface formed with filtrate grooves. The grooved filter surface of the diaphragm sheet defines at least one side of a filter chamber to which a prefilt is fed under pressure. The prefilt is filtered by a filter medium covering the diaphragm sheet. Subsequently a pressure fluid is supplied to a fluid chamber formed between the diaphragm sheet and the base plate, bulging the diaphragm sheet toward the filter chamber to press the cake remaining in the filter chamber. The filtrate filtered off by the filter medium flows down the filtrate grooves in the filter surface. Since a filtrate passage for drawing off the filtrate is formed in the base plate, the diaphragm plate is provided in its filter surface with a filtrate outlet communicating with the filtrate passage in the base plate.
FIG. 1 shows an expressing filter plate of the type described which comprises the combination of a diaphragm sheet A and a base plate C so as to effect both filtration and expression. With the illustrated construction, the diaphragm sheet A has a filtrate outlet B which is merely so formed as to communicate with a filtrate passage D formed in the base plate C. Expressing filter plates E are arranged alternately with ordinary filter plates G having filter surfaces F, or the same expressing filter plates are arranged side by side although unillustrated, with filter chambers H provided between adjacent plates for the filtration of prefilt and expression of cake. When expressing the cake, a pressure fluid is fed to a fluid chamber I between the diaphragm sheet A and the base plate C to establish a pressure balance between the chamber I and the filter chamber H. Consequently the diaphragm sheet A is usually subjected to pressure for example of about 4 to 10 kg/cm.sup.2 from both the chambers H and I on its opposite sides, with the result that the sheet tends to decrease in thickness and to spread out in a plane, undergoing plastic deformation. This deformation is directed concentrically to the filtrate outlet portion B where no resistance is encountered. The repetition of filtration and expression leads to increased deformation with the lapse of time, consequently blocking the filtrate outlet B. The filtrate outlet B is provided outside the bulging area of the diaphragm sheet A, namely in the portion of the sheet A which is clamped between the base plate C and the filter plate G or between the base plates, whereby the filtrate outlet portion B is held immediately adjacent to the filtrate passage D of the base plate. This construction readily ensures reliable communication between the filtrate outlet B and the filtrate passage D. However since the expressing filter plate E and filter plate G, or the closely fitted filter plates are tightly held together during expressing operation without any separation, these plates are pressed against each other with pressure greater than the expressing pressure. This produces greater deformation in the filtrate outlet portion B of the diaphragm A, promoting the blocking of the outlet B.
When the filtrate outlet B has been constricted or blocked up, the filtrate flowing down the grooves K in the filter surface J of the diaphragm sheet A is greatly hindered or prevented from flowing into the filtrate passage D of the base plate C by way of the outlet B. Discharge of the filtrate is impeded or prevented. As a result, abnormal high pressure will build up in the filter chamber H and pressure fluid chamber I and further in the prefilt feed passage, entailing the hazard of damage to the apparatus.
A wash liquor inlet, like the filtrate outlet, is further formed in the diaphragm sheet in communication with a wash liquor passage in the base plate for the reverse washing of the filter medium to remove the cake. With the known construction, the wash liquor inlet similarly tends to be constricted or blocked, in which event reverse washing can not be performed sufficiently or may be prevented, with abnormal high pressure created to damage the wash liquor feeding system.